The present invention relates to a magnetron apparatus for use in microwave ovens and the like, and a manufacturing method for the same.
The magnetron apparatus is a microwave oscillating tube which operates at a fundamental frequency of, for example, 2,450 MHz, and is used as a high frequency source in electric apparatuses using microwaves such as microwave heaters and microwave discharge lamps. A typical configuration of the magnetron apparatus is such that a cathode and an anode are disposed coaxially cylindrically. More specifically, the magnetron apparatus comprises a coiled cathode, an anode cylinder disposed with the cathode as the central axis, and plural anode segments radially arranged around the central axis in a space inside the anode cylinder for defining a resonant cavity. The magnetron apparatus further comprises a pair of magnetic pole pieces disposed at upper and lower open ends of the anode cylinder and magnetically associated with an annular permanent magnet, plural strap rings for electrically interconnecting the anode segments, and an antenna with one end connected to one of the anode segments for discharging microwaves.
In the above-mentioned magnetron apparatus, after the anode cylinder, the anode segments, the antenna, the strap rings and the magnetic pole pieces are integrally assembled as an anode assembly, the cathode is disposed in the central portion of the anode assembly. In the magnetron apparatus, as well known, the precision with which the components are assembled greatly influences the performance of the apparatus, and the arrangement of the plural anode segments for defining a desired resonant cavity inside the anode cylinder are particularly important. Therefore, it is a technical problem of the magnetron apparatus to coaxially and radially secure the plural anode segments with high precision so as to be equally spaced on the inner surface of the anode cylinder with a predetermined distance from the cathode.
As a conventional manufacturing method for the magnetron apparatus, a brazing and soldering method is known in which the anode segments are pressed against the inner surface of the anode cylinder by use of a temporary assembling pin and all the anode segments are secured to the inner surface at once with a brazing filler metal as disclosed in, for example, examined and published Japanese patent application TOKKO Sho 57-18823.
Hereinafter, the conventional magnetron apparatus and the manufacturing method will be described with reference to FIG. 16 and FIG. 17.
FIG. 16 is a partially cutaway perspective view showing a configuration of a principal part of an anode assembly in a conventional magnetron apparatus before a brazing filler metal is melted. FIG. 17 is a cross sectional view showing the configuration of the principal part of the anode assembly in the conventional magnetron apparatus after the brazing filler metal is melted.
As shown in FIG. 16 and FIG. 17, plural anode segments 52 (52a, 52b, 52c, 52d, as depicted in FIG. 16) are coaxially radially arranged inside an anode cylinder 51. Specifically, for example, ten anode segments 52 are equally spaced inside the anode cylinder 51. Each of the anode segments 52 is formed into a substantial rectangular shape having a longitudinal size of 9.5 mm and a lateral size of 13 mm, for example. In each of the anode segments 52, one end surface on the shorter side is secured to the inner surface of the anode cylinder 51. These anode segments 52 are pressed against the inner surface of the anode cylinder 51 by a jig pin 40, which is a temporarily used assembling pin, shown by the dash and dotted line of the figure, and the above-mentioned one end surface is secured to the inner surface of the anode cylinder 51 by melting a wire-form brazing filler metal 56 (FIG. 16).
When a non-illustrated coiled cathode is disposed along the central axis of the anode cylinder 51, each end surfaces of the anode segments 52 on the central side in the direction of the arrangement, i.e. an end surface each of the anode segments 52 opposed to the above-mentioned one end surface (hereinafter, the end surface on the central side will be referred to as an "inner end surface") is situated with a predetermined distance from the cathode, so as to define a desired resonant cavity inside the anode cylinder 51.
At opposite end surfaces (i.e., upper surface and lower surface) on the longer side of each of the anode segments 52, strap ring grooves 53a and 53b are provided for brazing two pairs of strap rings 54 (54a and 54b) and 55 (55a and 55b). At the upper end surface of each of the anode segments 52 where the strap ring groove 53a is provided, a terminal groove 53c is provided for connecting one end of a non-illustrated antenna.
The strap rings 54b and 55a are brazed to every two anode segments 52a, 52c, - - - , and the strap rings 54a and 55b are brazed to the remaining anode segments 52b, 52d, - - - . A plating layer (not shown) of the brazing filler metal 56 is formed on the surface of each of the strap rings 54 and 55, and when the brazing filler metal 56 is melted to secure the one end surfaces of the anode segments 52 to the inner surface of the anode cylinder 51, the plating layer is also melted, so that the strap rings 54 and 55 are secured to the corresponding anode segments 52.
The above-mentioned anode cylinder 51, anode segments 52, strap rings 54 and 55, and antenna (not shown) are made of, for example, oxygen free copper. The jig pin 40 is made of a metal member containing silicon nitride (Si.sub.3 N.sub.4), and the surface of a cylindrical portion which comes into contact with the inner end surface of each of the anode segments 52 is formed so as to be as smooth as the mirror finished surface. The brazing filler metal 56 is made of an alloy of silver and copper, and the strap rings 54 and 55 and the antenna (not shown) are made of copper having a silver plating layer provided on the surface thereof.
In such a conventional manufacturing method for the magnetron apparatus, first, the plural anode segments 52 and the strap rings 54 and 55 are placed in the respective positions inside the anode cylinder 51 by use of a non-illustrated temporary assembling jig. Then, the jig pin 40 is moved along the central axis of the anode cylinder 51 and press-fit from below into the central portion in the direction of the arrangement of the anode segments 52 (the central portion of the anode cylinder 51) as shown by the arrow Y of FIG. 16. So that the jig pin 40 contacts with the inner end surfaces of the anode segments 52. Thereby, the anode assembly is maintained in a preassembled condition where the one end surface each of the anode segments 52 are pressed against the inner surface of the anode cylinder 51 by the jig pin 40. Hereafter, only the temporary assembling jig is detached, and the brazing filler metal 56 is placed on the end surfaces on the longer side of the anode segments 52 so as to be in contact with the inner surface of the anode cylinder 51 as shown in FIG. 16. After one of the magnetic pole pieces (not shown) is attached to an upper open end of the anode cylinder 51, one end of the antenna (not shown) is attached to one of the anode segments 52. Then, the anode assembly in the preassembled condition is heated to a predetermined temperature (for example, 800 to 900.degree. C.) in a non-illustrated furnace. Thereby, the brazing filler metal 56 is melted and flows into a clearance between the inner surface of the anode cylinder 51 and the one end surface each of the anode segments 52 caused by expansion. At this time, the plating layers on the strap rings 54 and 55 and the antenna (not shown) are also melted. Hereafter, by taking the anode assembly out of the furnace while maintaining the preassembled condition, and cooling it, the inner surface of the anode cylinder 51 and the one end surface each of the anode segments 52, the strap ring grooves 53a and 53b and the corresponding strap rings 54 and 55, and the one of the anode segment 52 and the antenna (not shown) are secured.
Consequently, after the jig pin 40 is downwardly pulled out, the other of the magnetic pole pieces (not shown) is attached to a lower open end of the anode cylinder 51, and thereby the assembly of the anode assembly is finished.
In the conventional magnetron apparatus and the manufacturing method as described above, when the jig pin 40 is press-fit or taken out by moving it in the direction of the central axis, the jig pin 40 comes into contact with and rubs against the inner end surface of each of the anode segments 52 over the entire surface in the direction of the central axis. That is, in the conventional magnetron apparatus and the manufacturing method, the contact surface of the jig pin 40 and each the anode segments 52 equal the length of the inner end surface in the direction of the central axis, and the length of the contact surface (shown at A in FIG. 16) is long. For this reason, in the conventional magnetron apparatus and the manufacturing method, during the while the jig pin 40 is being press-fit or being taken out, contact pressure exerted on the anode segments 52 through the contact surfaces increases, so that the anode segments 52 are apt to be deformed. When such deformation is caused on the anode segments 52, the molten brazing filler metal 56 does not deposit onto the entire surface of the one end surface each of the anode segments 52 but the anode segments 52 come off due to insufficient brazing. Further, the deformation of the anode segments 52 changes the configuration of the strap ring grooves 53a and 53b, so that deformation of the strap rings 54 and 55 are caused and the strap rings 54 and 55 come off because the strap rings 54 and 55 are not secured to the strap ring grooves 53a and 53b.
When the components such as the plural anode segments 52 are mass-produced, it is difficult to form these components so as to have uniform outer dimensions and it is impossible to completely prevent the outer dimensions from varying. For this reason, in the conventional magnetron apparatus and the manufacturing method, there are occasions when the anode and the cathode are short-circuited because of the variation in outer dimension. Specifically, in the case that the outer dimensions of the anode segments 52 are greater than predetermined outer dimensions and the outer dimensions of the inner surface of the anode cylinder 51 are smaller than predetermined outer dimensions, when the jig pin 40 is press-fit from below, the inner end surface each of the anode segments 52 is extended in the movement direction of the jig pin 40 by stress caused by the press fitting of the jig pin 40, so that copper foil burrs 57 as illustrated in FIG. 18 are caused at the upper end of the inner end surface. As a result, when the cathode is placed along the central axis of the anode assembly (anode cylinder 51), it often happens that the burrs 57 come into contact with the cathode and the contact causes a short circuit. Further, in the case that the anode cylinder 51 or the anode segments 52 are formed to have outer dimensions which are different from predetermined outer dimensions as mentioned above, greater power is necessary when the jig pin 40 is press-fit or taken out, thus resulting in dents and scratches on the jig pin 40 that require the jig pin 40 to be replaced.
Further, in each of the anode segments 52, as has been explained in the above, the strap ring groove 53a and the terminal groove 53c are provided at one of the end surface on the longer side, and the strap ring groove 53b is provided at the other end surface. For this reason, in the conventional magnetron apparatus and the manufacturing method, when the jig pin 40 is press-fit so as to be in contact with the inner end surface each of the anode segments 52, the pressing force which the anode segments 52 receive from the jig pin 40 and the anode cylinder 51 is not uniform in the direction of the central axis. Specifically, when each anode segment 52 is divided into three areas, for example, an upper area Va, a central area Vb and a lower area Vc in the direction of the central axis as shown in FIG. 17, the central area Vb does not include the grooves 53a, 53b and 53c. Therefore, the pressure exerted on the central area Vb is greater than that exerted on the upper and lower areas Va and Vc. When the anode assembly in the preassembled condition is heated, since the anode segments 52 expand and the molten brazing filler metal 56 flows into the clearance between the anode cylinder 51 and the anode segments 52, the pressing force applied on the upper and lower areas Va and Vc by the jig pin 40 is smaller than the pressing force which the central area Vb receives therefrom.
Thus, when the pressing force exerted on the anode segments 52 is not uniform in the direction of the central axis, because of the above-mentioned reasons combined with the fact that the surface of the jig pin 40 is as smooth as the mirror finished surface, the anode segments 52 slide over the inner surface and are secured to the inner surface of the anode cylinder 51 with the one end surfaces of the anode segments 52 being inclined from the direction of the central axis. Consequently, in the conventional magnetron apparatus and the manufacturing method, the distance between two adjoining anode segments 52, i.e. the pitch varies as shown at P1, P2 and P3 in FIG. 19, so that the plural anode segments 52 are not equally spaced inside the anode cylinder 51.
As has been explained above, in the conventional magnetron apparatus and the manufacturing method, deformation of the anode segments 52 and the strap rings 54 and 55 and coming-off of brazed parts due to insufficient brazing are apt to occur, and the burrs 57 and the variation in pitch of the plural anode segments 52 result therefrom. Therefore, in the conventional magnetron apparatus and the manufacturing method, it has been impossible to define the desired resonant cavity inside the anode assembly 51, so that it is impossible to oscillate microwaves of the fundamental frequency with stability. Further, the magnetron efficiency deteriorates and high-frequency noises are markedly generated.
Examples of a conventional magnetron apparatus intended for reducing the contact pressure between the jig pin 40 and the anode segments 52 include one disclosed in unexamined and published Japanese patent application TOKKAI Sho 64-52365. In the conventional magnetron apparatus, by forming the cylindrical portion of the jig pin 40 so as to have dimensions which are 50 to 70% of the inner end surface each of the anode segments 52, the contact pressure is reduced which is caused when the jig pin 40 is press-fit or taken out.
However, in the conventional magnetron apparatus, when the anode segments 52 are pressed against the inner surface of the anode cylinder 51, on the inner end surface each of the anode segments 52 there are produced one area which is pressed by being in contact with the cylindrical portion of the jig pin 40 and the other area which is not pressed because it does not come into contact with the cylindrical portion. Thereby, in the conventional magnetron apparatus, the pressing force which the anode segments 52 receive is unbalanced in the direction of the central axis, so that in addition to the problem that the anode segments are not equally spaced, a new problem arises that the diameter of an inscribed circle defined by the inner end surface each of the plural anode segments 52 varies in the direction of the central axis (the vertical direction). Because of these problems, the conventional magnetron apparatus is not realized and commercialized.